Flat plate collector and solar water heater

A flat plate collector (FPC) absorbs both beam and diffuse radiation on a flat surface and transfers the heat to a fluid, without concentrating it. It is used for temperatures up to about 80–100 °C.

Construction

• Absorber plate: a copper or aluminium sheet with tubes (risers) attached to it, coated with a black or selective coating (high absorptance, low emittance).
• Transparent cover: one or two glass sheets; they pass short-wave solar radiation but block long-wave re-radiation from the plate (greenhouse effect) and reduce convection loss.
• Insulation: glass wool or polyurethane (50–75 mm) at the back and sides.
• Casing (box): aluminium or galvanised steel, weatherproof.
• Headers: connect the riser tubes to the inlet and outlet pipes.

   sun rays \ \ \
 ===== glass cover ======
   air gap
 ~~~ absorber plate + tubes ~~~
 ///// insulation //////////////
 |________ casing ____________|

Working

Radiation passes through the glass and heats the plate. Water in the tubes takes up this heat and leaves hotter. The useful heat is given by the Hottel-Whillier-Bliss equation $Q_u = A_c F_R\,[S - U_L (T_i - T_a)]$, and the efficiency is $\eta = Q_u/(A_c G_T)$.

Thermosyphon water heater

                   hot water to taps
                          ^
   cold mains     +-------|-------+
   -------------->|  storage tank |
                  |  (hot at top) |
                  +---------------+
            hot   ^             |  cold
            water |             |  water
            rises |             v  falls
                +-----------------+
                |  flat plate     |
                |  collector      |
                +-----------------+
         (collector placed below the tank)

The storage tank is placed above the collector. Water heated in the collector becomes less dense and rises to the top of the tank, while the colder, denser water from the bottom of the tank flows down to the collector inlet. This natural circulation (thermosyphon) needs no pump and runs only while the sun shines. The bottom of the tank should be at least 0.3–0.5 m above the top of the collector, and the pipes must be short and sloped, without air traps.